semconvgen: Add a flag for a path to a custom capitalizations file

[carrbs]

The capitalizations slice in the semconvgen generator.go file does not include some of the keywords we would like capitalized for the generated metrics in semconv, namely:

• ASPNETCore
• JVM

Adding a new optional flag (e.g. capitalizations-path) and appending the strings from the capitalizations file (file type to be determined) to the existing capitalizations slice would provide the flexibility to add these strings (and others, as necessary) without modifying the code in the generator file.

A usage example:

metric-capitalizations.txt

ASPNETCore
JVM

Invoking semconvgen with the new flag:

semconvgen -i "/path/to/semconv/yaml" \
--only=metric  \
-f metric.go \
-t "/path/to/jinja/template" \
--capitalizations-path="/path/to/metric-capitalizations.txt" \
-s "someVersion"