Add new proxy types

[camfairchild]

Description

This PR adds two new proxy types: Transfer and SmallTransfer.

The Transfer proxy allows only Balances::transfer_* extrinsics including keep alive, allow death, and transfer all.

The SmallTransfer proxy only allows transfers for amounts below SMALL_TRANSFER_LIMIT which is set to 0.5 TAO

These new proxy types help to make key management more flexible. By allowing more proxy types such as these one can separate the responsibility of different keys instead of using the parent key, potentially exposing all operations.

An example setup is to have key A use the existing staking proxy type and key B use the transfer type. And then if A becomes compromised an attacker may only stake/unstake. If B becomes compromised, an attacker may only transfer free-balance (i.e. excluding any stake). Thereby reducing the attack surface, while allowing keys to be used in otherwise "hot" deployments.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run cargo fmt and cargo clippy to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Additional Notes

Please provide any additional information or context that may be helpful for reviewers.

[camfairchild]

Needs spec bump