chore: Getter for Request Handler (#645)

<!--
We appreciate the effort for this pull request but before that please
make sure you read the contribution guidelines, then fill out the blanks
below.

Please format the PR title appropriately based on the type of change:
  <type>[!]: <description>
Where <type> is one of: docs, chore, feat, fix, test, misc.
Add a '!' after the type for breaking changes (e.g. feat!: new breaking
feature).

**All third-party contributors acknowledge that any contributions they
provide will be made under the same open-source license that the
open-source project is provided under.**

Please enter each Issue number you are resolving in your PR after one of
the following words [Fixes, Closes, Resolves]. This will auto-link these
issues and close them when this PR is merged!
e.g.
Fixes #1
Closes #2
-->

# Fixes #

Set getter for Request Handler

### Checklist
- [x] I acknowledge that all my contributions will be made under the
project's license
- [ ] Run `make test-docker`
- [ ] Verify affected language:
- [ ] Generate [twilio-go](https://github.com/twilio/twilio-go) from our
[OpenAPI specification](https://github.com/twilio/twilio-oai) using the
[build_twilio_go.py](./examples/build_twilio_go.py) using `python
examples/build_twilio_go.py path/to/twilio-oai/spec/yaml
path/to/twilio-go` and inspect the diff
    - [ ] Run `make test` in `twilio-go`
    - [ ] Create a pull request in `twilio-go`
    - [ ] Provide a link below to the pull request
- [ ] I have made a material change to the repo (functionality, testing,
spelling, grammar)
- [ ] I have read the [Contribution
Guidelines](https://github.com/twilio/twilio-oai-generator/blob/main/CONTRIBUTING.md)
and my PR follows them
- [ ] I have titled the PR appropriately
- [ ] I have updated my branch with the main branch
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have added the necessary documentation about the functionality
in the appropriate .md file
- [ ] I have added inline documentation to the code I modified

If you have questions, please create a GitHub Issue in this repository.

Author: manisha1997

src/main/resources/twilio-go/api_service.mustache

@@ -10,6 +10,10 @@ type ApiService struct {
 	requestHandler *twilio.RequestHandler
 }
 
+func (c *ApiService) RequestHandler() *twilio.RequestHandler {
+  return c.requestHandler
+}
+
 func NewApiService(requestHandler *twilio.RequestHandler) *ApiService {
 	return &ApiService {
 		requestHandler: requestHandler,