CLI tool: Support constructing transactions

[jsdw]

With scale_value, we can parse scale_value::Values from the commandline.

As a bit of a proof of concept that we can build on, and once #581 has landed, let's give the CLI tool the ability to look up available pallets/calls, and build up a transaction by making a call and passing Values as the parameters to it.

Loads of future bits we can add onto this, but I think this would be a great start to testing the viability of the dynamic support.

[jsdw]

As a note; one of the eventual goals from here is to allow transactions to be built using the CLI tool and then, if desired, signed by navigating to a URL and using some hardware wallet to do the signing in JS, then handing back the signed transaction to the CLI tool to finish submitting it.

[jsdw]

To elaborate on this, examples of interacting with this command might look something like:

> subxt tx
Usage: subxt tx <pallet> <call> [argument...]

Available pallets:
    Auctions
    Authorship
    Babe
    Balances

> subxt tx balances
Usage: subxt tx balances <call> [argument...]

Available calls:
    forceTransfer
    forceUnreserve
    setBalance
    transfer

> subxt tx balances transfer
Usage: subxt tx balances transfer [option] <dest> <value>

Options:
    --file
        Location of metadata file
    --url
        URL to node
    --unsigned
        Construct an unsigned call and return the hex payload
    --details
        Print details about this call

> subxt tx balances transfer --details
Transfer some liquid free balance to another account.

This call has the following arguments:
    dest
    value

The 'dest' argument has the type:

enum MultiAddress {
    Id([u8; 32])
    Index(u32)
    Raw([u8])
}

Examples of 'dest':

'Id ((0, 1, 2, 3, 4, 5, 6, 7, 8))'
'Index (12345)'
'Raw((1,2,3,4))'

The 'value' argument has the type:

u128

Examples of 'value':

0
123456

> subxt tx balances transfer --unsigned --file metadata.scale 'Id ()'
Usage: subxt tx balances transfer <dest> <value>

The following arguments were not provided:
- value

> subxt tx balances transfer 'Id ()' 123456
Usage: subxt tx balances transfer <dest> <value>

Argument 'dest' has the wrong shape.

The 'dest' argument has the type:

enum MultiAddress {
    Id([u8; 32])
    Index(u32)
    Raw([u8])
}

Examples of 'dest':

'Id ((0, 1, 2, 3, 4, 5, 6, 7, 8))'
'Index (12345)'
'Raw((1,2,3,4))'

> subxt tx balances transfer --unsigned --file metadata.scale 'Id ((0, 1, 2, 3, 4, 5, 6, 7, 8))' 123456
0x1234ebcd

This feels like a good first step, and would allow us to inspect the metadata or node to learn about the available calls and their types, and construct an unsigned extrinsic.

We can later add options to also sign and submit an extrinsic potentially.

There are already several challenges in giving the above, and I could see functions like this being useful:

// Take some SCALE type ID and registry and pretty print out the type information for it
// (this can be more detailed than the 'scale-value::Value' representation cares about, so that we can
// see eg the expected size of the array or whatever where appropriate. Prob best to print something that
// looks like Rust code; should be fairly simple anyway and no generics etc)
fn print_type_description(type_id: u32, registry: &PortableRegistry) -> String

// Take some SCALE type ID and registry and pretty print one or more examples for it. For enum types we
// could print out up to eg 5 variants, and for primitive types we can print some sensible values, maybe the 
// high, low and some random middle val.
fn print_type_example(type_id: u32, registry: &PortableRegistry) -> impl Iterator<Item=String>

Any response that counts as an error (ie is emitted because the user entered incorrect details) should be emitted to stderr, else emit responses to stdout.

(Note to self: it'd be great if scale-value supported interpreting hex values into composite vecs of bytes, so eg 0x01FF would be equivalent to the composite type (1, 255); this would make passing in acocunt IDs and such nicer, since we often get them as hex and will return them as hex).