Use bitcoin's base64 feature for Psbts

[RCasatta]

Description

bitcoin 0.26.2 comes with a new base64 feature converting PSBT from/to string, this PR start using this feature.

Even though we use base64 only in tests (base64 is a dev-dep) downstream users will gain the advantage of not having to use an additional base64 dep to parse PSBTs

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[afilini]

If you want to use a feature that is only available from 0.26.2 I think you'll have to change the version accordingly. I think ~0.26.2 is what we want here: https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#tilde-requirements

[jb55]

I get this error after this change:

    Updating git repository `https://github.com/bitcoindevkit/bdk`
    Updating crates.io index
error: failed to select a version for `bitcoin`.    ... required by package `electrum-client v0.7.0`
    ... which is depended on by `bdk v0.8.1-dev (https://github.com/bitcoindevkit/bdk?rev=9ff86ea37c2ecd99cea9f9713d73b64fefc94616#9ff86ea3)`
    ... which is depended on by `btcmerchant v0.1.0 (/home/jb55/src/rust/btcmerchant/btcmerchant)`
    ... which is depended on by `btcmerchant-server v0.1.0 (/home/jb55/src/rust/btcmerchant/btcmerchant-server)`
versions that meet the requirements `=0.26.0` are: 0.26.0

all possible versions conflict with previously selected packages.

  previously selected package `bitcoin v0.26.2`
    ... which is depended on by `bdk v0.8.1-dev (https://github.com/bitcoindevkit/bdk?rev=9ff86ea37c2ecd99cea9f9713d73b64fefc94616#9ff86ea3)`
    ... which is depended on by `btcmerchant v0.1.0 (/home/jb55/src/rust/btcmerchant/btcmerchant)`
    ... which is depended on by `btcmerchant-server v0.1.0 (/home/jb55/src/rust/btcmerchant/btcmerchant-server)`

failed to select a version for `bitcoin` which could resolve this conflict

looks like electrum-client needs to be bumped as well?

[notmandatory]

Which features are you using? the CI already checks all the features separately, and I confirmed on my local system (rust 1.51.0) bdk builds fine with the default features ("key-value-db", "electrum"). I also did a cargo update and still building OK.