feat: blockstore: GetMany blockstore method

[i-norden]

This PR introduces a GetMany blockstore method to compliment the existing PutMany method. This is for use in a GetMany go-ipld-cbor datastore implementation which, in turn, is for use in parallel ForEach methods for both go-amt-ipld and go-hamt-ipld. This PR uses the TxnDatastore keytransform/namespace wrapper introduced in https://github.com/ipfs/go-datastore/pull/210.

GetMany go-ipld-cbor PR that uses this: https://github.com/ipfs/go-ipld-cbor/pull/97

TODO: Replace replace directives if/when the dependency is merged and released

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 42 lines in your changes are missing coverage. Please review.

Project coverage is 65.64%. Comparing base (0a566c9) to head (b3ed048). Report is 124 commits behind head on main.

@@            Coverage Diff             @@
##             main     #492      +/-   ##
==========================================
- Coverage   65.78%   65.64%   -0.15%     
==========================================
  Files         207      203       -4     
  Lines       25156    25385     +229     
==========================================
+ Hits        16549    16663     +114     
- Misses       7147     7235      +88     
- Partials     1460     1487      +27     

Files	Coverage Δ
blockstore/blockstore.go	54.95% <50.00%> (-3.02%)	:arrow_down:

... and 42 files with indirect coverage changes