feat(teler): add `CaddyHandler{,FuncWithNext}`

[dwisiswant0]

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Summary

Adds

• CaddyHandler method,
• CaddyHandlerFuncWithNext method.

Proposed of changes

This PR fixes/implements the following bugs/features:

• Bug 1
• Bug 2
• Feature 1
• Feature 2
• Breaking changes

How has this been tested?

Proof:

Closing issues

Fixes #

Checklist:

• [x] My code follows the code style of this project.
• [x] My changes successfully ran and pass linters locally (run make lint).
• [x] I have written new tests for my changes.
  • [x] My changes successfully ran and pass tests locally.
• [x] My change requires a change to the documentation.
  • [x] I have updated the documentation accordingly.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 53.33333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 78.27%. Comparing base (5974890) to head (192180e). Report is 3 commits behind head on master.

Files	Patch %	Lines
handler.go	53.33%	5 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #199 +/- ## ========================================== - Coverage 78.67% 78.27% -0.41% ========================================== Files 12 12 Lines 933 948 +15 ========================================== + Hits 734 742 +8 - Misses 132 137 +5 - Partials 67 69 +2 ```

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