example: Fix wallet examples

[ValuedMammal]

These are some fixes that can be implemented for the wallet_* examples in the example-crates dir.

• Print balances either in BTC or sats (but not both)
• wallet_esplora_async appears to be printing the KeychainKind twice, and forgets to print spk0
• Use tempfile instead of std::env::tempdir, as this should automatically clean up temporary files
• Consider changing wallet examples to use signet and possibly a different server URL to fix rate limiting issues
• Try to recycle the sats so that running an example doesn't result in a net loss of "funds"

[storopoli]

You can assign this to me.
I am eager to help.

[storopoli]

@ValuedMammal the problem with

• Consider moving these smaller examples to the bdk/examples folder

is that we cannot add them to the wallet/examples folder is due to the fact that bdk_esplora and bdk_electrum crates are not dependencies for bdk_wallet.
Hence we cannot add them to the cargo examples.

However we can circumvent this by adding both bdk_electrum and bdk_esplora to the dev-depencies of bdk_wallet.
Which I think is not worth it the extra overhead.

Source: https://users.rust-lang.org/t/cargo-example-only-dependencies/3215

[ValuedMammal]

we cannot add them to the wallet/examples folder is due to the fact that bdk_esplora and bdk_electrum crates are not dependencies for bdk_wallet.

you're right, they will probably have to stay in example-crates