Fix/policy path multisig

[afilini]

Description

Opening against the relase branch, as this is a bugfix instead of a new feature. Once the release is done it'll get merged back into master.

[policy] Allow specifying a policy path for Multisig

While technically it's not required since there are no timelocks inside, it's still less confusing for the end user if we allow this instead of failing like we do currently.

[policy] Remove the TooManyItemsSelected error

The TooManyItemsSelected error has been removed, since it's not technically an error but potentailly more of an "over-constraint" over a tx: for instance, given a thresh(3,pk(a),pk(b),older(10),older(20)) descriptor one could create a spending tx with the [0,1,2] items that would only be spendable after 10 blocks, or a tx with the [0,2,3] items that would be spendable after 20.

In this case specifying more items than the threshold would create a tx with the maximum constraint possible, in this case the 20 blocks. This is not necessarily an error, so we should allow it without failing.

Checklists

All Submissions:

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

New Features:

• I've added tests for the new feature
• I've added docs for the new feature
• I've updated CHANGELOG.md

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

ACK b61427c

Tested with bdk-cli and problem resolved.

Test steps:

1. in local repo bdk-cli master edit Cargo.toml

   ...
   [dependencies] 
   bdk = { git = "https://github.com/afilini/bdk.git", branch = "fix/policy-path-multisig", default-features = false, features = ["all-keys"]}
   bdk-macros = { git = "https://github.com/afilini/bdk.git", branch = "fix/policy-path-multisig" }
   ...
   

2. run bdk-cli repl command that failed for @thunderbiscuit. It now does not throw an error:

   cargo run --all-features -- repl --descriptor "wsh(multi(2,tprv8ZgxMBicQKsPdxAx3PFTiciVrm1rpnUCXjhbNheQhtsMQUnr8MNb3KWJEJ4Bp6EcTZSSKkvBt5HoRkv3aW7oyowy1BKEo5GcjZXHhyWSHPa/84h/1h/0h/0/*,[6bf3c709/84'/1'/0']tpubDCujUzKmyZPsAxjMyT3cVWCKRAPTtjJbJjez27kn1YZPPaSwbdeXXs9ZGkfE1rXRm4uRzfyXztMhb8ZNDS43mtwJbGbWQL8M4tY6y79hbSq/0/*))"
   

   create_tx --to "tb1qtdhckkuwl55598u6vgm2stvj4xgurzl4f35gck:0" --send_all --external_policy '{"nu9zsur5": [0,1]}'
   {
     "details": {
       "fees": 138,
       "height": null,  
       "received": 0,
       "sent": 6085,
       "timestamp": 1613246811,
       "transaction": null,
       "txid": "462dbb9242329f2826da3b9338802747efd2e90b8b042ec2ce7c1e289b07f0cc"
     },
     "psbt": "cHNidP8BAFIBAAAAAXOmXM5TZ1v2bujTTd3/va3IJcjfeEN+qF+6miF7KIrfAAAAAAD/////ATsXAAAAAAAAFgAUW2+LW479KUKfmmI2qC2SqZHBi/UAAAAAAAEBK8UXAAAAAAAAIgAg35WIXWE2CCtdwFkCbfFFKhKm3Xzx1f6rTq6WJpb1qy8BBUdSIQL9Oynd2xz0dqvB9a0+9c6pLyPO2655NSY59/5MT8WfvSEDmPCPGwmVd+O8bJb5WGhLLRd1PgMhG7Ipu1SXdVDko0ZSriIGAv07Kd3bHPR2q8H1rT71zqkvI87brnk1Jjn3/kxPxZ+9GDrrnyZUAACAAQAAgAAAAIAAAAAAAAAAACIGA5jwjxsJlXfjvGyW+VhoSy0XdT4DIRuyKbtUl3VQ5KNGGGvzxwlUAACAAQAAgAAAAIAAAAAAAAAAAAAA"
   }