feat: in-memory provider for sdk testing

[Kavindu-Dodan]

This PR

Is related to OFEP open-feature/ofep#71 and shows the feasibility of an in-memory provider

Features

• Provider is constructed with map of pre-defined flags [1]
• Supports context evaluations based on callbacks [2]

[1] - open-feature/ofep#71
[2] - https://github.com/open-feature/go-sdk/pull/192/files#diff-1e56a5f9a72a9694aeb6f3510e8a2e02359c3cbdb9bd5c906219e30756d857efR181

[codecov]

Codecov Report

Merging #192 (94bec17) into main (2bc3c38) will decrease coverage by 0.36%.
The diff coverage is 76.33%.

@@            Coverage Diff             @@
##             main     #192      +/-   ##
==========================================
- Coverage   78.99%   78.63%   -0.36%     
==========================================
  Files           9       10       +1     
  Lines        1076     1203     +127     
==========================================
+ Hits          850      946      +96     
- Misses        204      230      +26     
- Partials       22       27       +5     

Files Changed	Coverage Δ
pkg/openfeature/memprovider/in_memory_provider.go	75.59% <75.59%> (ø)
pkg/openfeature/event_executor.go	98.71% <100.00%> (ø)

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[toddbaert]

This is very straightforward. It's pretty much convinced me we should do this everywhere.

[beeme1mr]

Perhaps we should add the e2e folder to the Codecov ignore list.

[Kavindu-Dodan]

Perhaps we should add the e2e folder to the Codecov ignore list.

For this PR, coverage complaint is on newly added in-memory providers. Some of the error paths are not covered with tests but IMO these are not essential (ex:- testing errors for each flag type)