Add optional display name to metadata.yaml

[mx-psi]

From @jaydeluca in #otel-ecosystem-explorer (ref):

i'm curious what you would think about the feasibility of introducing an optional "display_name" field to the collector metadata components? for example, the basicauthextension could be "Basic Authenticator" like in the readme. We could always map this in our automation, but the less transforms the better in terms of keeping complexity down

I think this makes sense and can eventually be used in the README title. As a first step we can just add the field.

[thc1006]

Dear @mx-psi,
Can let me try to implement this feature?

[mx-psi]

@thc1006 Sure!

Here is how I would go about it:

1. Add the field to this struct:

   opentelemetry-collector/cmd/mdatagen/internal/metadata.go

   Line 18 in 82f2534

   type Metadata struct {

2. Add a section to the template here: https://github.com/open-telemetry/opentelemetry-collector/blob/main/cmd/mdatagen/internal/templates/readme.md.tmpl that checks if the field is set, and if so, adds a title with the display nam
3. Add tests!
4. Try it out with the OTLP receiver by modifying its metadata.yaml file, removing the first two lines from its README.md file and running make gogenerate to re-generate the documentation