Add SPV proof option for Anchors. Differentiate anchors, extra-tx witnesses and tx witnesses.

[dr-orlovsky]

Implements RGB-WG/RFC#3 at the level of BP Core.

This would allow to validate single-use seals without Internet connectivity and bitcoin blockchain indexer.

[codecov]

Codecov Report

Attention: Patch coverage is 17.14286% with 29 lines in your changes are missing coverage. Please review.

Project coverage is 47.4%. Comparing base (7b1ff00) to head (7aece00).

❗ Current head 7aece00 differs from pull request most recent head 5b93351. Consider uploading reports for the commit 5b93351 to get more accurate results

Files	Patch %	Lines
dbc/src/anchor.rs	0.0%	22 Missing ⚠️
consensus/src/block.rs	0.0%	4 Missing ⚠️
seals/src/txout/witness.rs	0.0%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #83     +/-   ##
========================================
- Coverage    48.2%   47.4%   -0.9%     
========================================
  Files          36      34      -2     
  Lines        3290    3174    -116     
========================================
- Hits         1587    1503     -84     
+ Misses       1703    1671     -32     

Flag	Coverage Δ
rust	47.4% <17.1%> (-0.9%)	⬇️

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[dr-orlovsky]

Closing since it was decided that SPV proofs are not a consensus-level thing and must be added only in the standard library.