Add means to retrieve more birthday height values

[AArnott]

• Optimized birthday height: the block number on the first block that actually contains a transaction related to this account. A user may create a wallet at block height X, but only receive ZEC for the first time at X+Y, at that point, only X+Y is relevant as a birthday height for that account.
• Rebirth height: The height of the first block that contains a transaction that provides unspent funds (whether as a shielded note or UTXO). This could be provided as a birthday height when importing the account in order to access the full balance of the account, but not necessary download its full history.

[codecov]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Comparison is base (592417e) 75.46% compared to head (1f59fa8) 75.23%.

Files	Patch %	Lines
src/Nerdbank.Zcash/RustBindings/LightWallet.cs	0.00%	26 Missing ⚠️
src/Nerdbank.Zcash/LightWalletClient.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
- Coverage   75.46%   75.23%   -0.23%     
==========================================
  Files         155      155              
  Lines        9043     9070      +27     
  Branches      830      832       +2     
==========================================
  Hits         6824     6824              
- Misses       2018     2045      +27     
  Partials      201      201              

Flag	Coverage Δ
LinuxHost	75.23% <0.00%> (?)
Release	75.23% <0.00%> (-0.23%)	⬇️
WindowsHost	75.23% <0.00%> (-0.23%)	⬇️
macOSHost	75.23% <0.00%> (-0.23%)	⬇️

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