feat: add custom initialisms support to gen package and avrogen

[hhromic]

Hello, this simple PR adds support for custom initialisms to the gen package and avrogen command.

Additional custom initialisms are necessary because strcase.ToGoPascal only uses GoLint's list of initialisms.
Currently, if an initialism in struct/field names is not part of Golint's, then strcase wrongly downcases it.

For example, consider the following schema containing the domain initialism AID:

{
  "type": "record",
  "name": "AIDEventHTTPService",
  "namespace": "example",
  "fields" : [
    {"name": "AIDNumber", "type": "long"},
    {"name": "Desc", "type": "string"}
  ]
}

Currently, avrogen generates the following Go struct (struct/field names: AID -> Aid):

package example

// Code generated by avro/gen. DO NOT EDIT.

// AidEventHTTPService is a generated struct.
type AidEventHTTPService struct {
        AidNumber int64  `avro:"AIDNumber"`
        Desc      string `avro:"Desc"`
}

With this PR, -initialisms AID can be given to avrogen to generate the correct struct/field names:

package example

// Code generated by avro/gen. DO NOT EDIT.

// AIDEventHTTPService is a generated struct.
type AIDEventHTTPService struct {
        AIDNumber int64  `avro:"AIDNumber"`
        Desc      string `avro:"Desc"`
}

There is a related PR in the upstream strcase repository: ettle/strcase#13
That PR allows to preserve any extra initialisms without having to define them in advance, however the maintainers seems to not be merging it any time soon. This PR allows to solve the issue in the mean time.

[nrwiersma]

One nitpick about test error assertion, otherwise looks good.

[hhromic]

One nitpick about test error assertion, otherwise looks good.

Thanks for the feedback, indeed looks cleaner after your suggestion.

I merged main too into this branch. Thanks for being open to contributions! Appreciated!

[nrwiersma]

LGTM 🎉