feat: extend with external packages and name regexp

[nissim-natanov-outreach]

Hey Jannis, thanks for building this wonderful tool!

We would love to use the goverter in our projects in Outreach, and we already know we need to pass a LOT of extend methods in many places to enable custom conversions. The goal of this PR is to enable consolidation of multiple extend conversion methods in a set of dedicated 'packages', then passing this set as a one-line extend to the goverter by invoking goverter as a lib from inside our own codegen tool (or via goverter's command line - we have not decided yet).

This PR enables external packages and name regexp usage in the extend statements.

• extend with method name and external package goverter:extend github.com/google/uuid:FromBytes
• extend with method name regexp, external package (golang style regexp pattern) goverter:extend strconv:Parse.*
• extend with method name regexp, local package goverter:extend MyStringTo.*
• existing extend with name only did not change goverter:extend MyStringToInt
• update readme with new examples
• update unit tests to include sunny day and error conditions
• allow extends to be provided via command line, very useful if goverter is invoked from another codegen, knowing that 99% of covered projects will always need set of conversion methods.

[nissim-natanov-outreach]

@jmattheis Janis, I was not able to add you as a code reviewer (option grayed out), hoping to grab your attention via mention here :).

[nissim-natanov-outreach]

Hey @nissim-natanov-outreach, thanks for your contribution. This is pretty cool, I like it (:. I mostly remarked error messages, and I think some test cases could be added. See sub comments.

Thanks!! I will be working on addressing the feedback today, hope to get new iteration out soon.

[nissim-natanov-outreach]

thanks a ton fro the feedback, comments addressed :), PTAL

[codecov-commenter]

Codecov Report

Merging #14 (6622c3c) into main (29ad199) will decrease coverage by 0.39%. The diff coverage is 90.56%.

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
- Coverage   93.77%   93.38%   -0.40%     
==========================================
  Files          20       22       +2     
  Lines         932     1043     +111     
==========================================
+ Hits          874      974     +100     
- Misses         43       52       +9     
- Partials       15       17       +2     

Impacted Files	Coverage Δ
cmd/goverter/main.go	0.00% <0.00%> (ø)
example/mismatched/input.go	100.00% <ø> (ø)
generator/generator.go	97.26% <ø> (+0.46%)	:arrow_up:
generator/packages.go	91.66% <91.66%> (ø)
generator/extends.go	94.95% <94.95%> (ø)
generator/generate.go	87.87% <100.00%> (+1.21%)	:arrow_up:
runner.go	89.47% <100.00%> (+0.58%)	:arrow_up:

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