Rename tails variables in prover code

[Patrik-Stas]

• This is breaking change for provers constructing "selected credentials" datastructure when creating proof. While previously they had option to specify field tails_file, now they have to use tails_dir. It's essentially forced renamed, semantics didn't change. Previously tails_file attribute was actually expected to point to a directory (containing tails file)
  (eg. see https://github.com/hyperledger/aries-vcx/pull/901/files#diff-7542b0e9db422e41b81fc4f3bf5da974a9e5ec73a1a6a2d08d4698d4a9eb9b89R26 as example of prover accommodating this breaking change)

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@349d82e). Click here to learn what that means.
The diff coverage is 21.05%.

❗ Current head 3e84c41 differs from pull request most recent head fb1fecc. Consider uploading reports for the commit fb1fecc to get more accurate results

@@           Coverage Diff           @@
##             main     #901   +/-   ##
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  Coverage        ?   43.37%           
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  Files           ?      438           
  Lines           ?    34777           
  Branches        ?     7589           
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  Hits            ?    15085           
  Misses          ?    15231           
  Partials        ?     4461           

Flag	Coverage Δ
unittests-aries-vcx	43.37% <21.05%> (?)

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Impacted Files	Coverage Δ
aries_vcx/src/handlers/proof_presentation/types.rs	46.80% <ø> (ø)
aries_vcx/tests/utils/scenarios.rs	83.87% <0.00%> (ø)
...es_vcx/src/common/proofs/prover/prover_internal.rs	27.27% <22.22%> (ø)

[gmulhearn]

nice, has been a smell for me for awhile

[gmulhearn]

out of sync with main, but LGTM