esplora indexer: detect mempool TXs

[zoedberg]

Closes RGB-WG/rgb-std#266 by allowing detection of mempool TXs from the esplora indexer. This way electrum and esplora indexers will behave the same.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 3.1%. Comparing base (f820fac) to head (36c932b).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/indexers/esplora_blocking.rs	0.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #241     +/-   ##
=======================================
- Coverage     3.1%   3.1%   -0.0%     
=======================================
  Files          14     14             
  Lines        1728   1731      +3     
=======================================
  Hits           53     53             
- Misses       1675   1678      +3     

Flag	Coverage Δ
rust	3.1% <0.0%> (-<0.1%)	⬇️

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