eventually: add 'postgres' feature flag

[ar3s3ru]

Expose the eventually-postgres crate through the postgres feature flag in the eventually public API crate.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

No Coverage information
No Duplication information

[codecov]

Codecov Report

Merging #98 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #98   +/-   ##
=======================================
  Coverage   53.72%   53.72%           
=======================================
  Files          14       14           
  Lines         711      711           
=======================================
  Hits          382      382           
  Misses        329      329           

Flag	Coverage Δ
#unit	53.72% <ø> (ø)

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Impacted Files	Coverage Δ
eventually-postgres/src/lib.rs	0.00% <ø> (ø)

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