Improve certificate handling in MQTT config flow

[jbouwh]

Proposed change

This PR will allow more type of certificates to be supplied through upload. The MQTT config flow will automatically convert the certificate tiles to the PEM format, to be stored as entry data.

File	Currently supported	Intended support
CA certificate file	PEM format only	PEM format and DER encoded certificate files
Client certificate file	PEM format only	PEM format and DER encoded certificate files
Client key file	PEM format (no password)	PEM format and DER encoded key files with or without password encryption

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue: MQTT Custom CA config flow fails unexpectedly #137060
• Link to documentation pull request: Improved MQTT broker certificate configuration home-assistant.io#37277
• Link to developer documentation pull request:
• Link to frontend pull request:

Checklist

• The code change is tested and works locally.
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[home-assistant]

Hey there @emontnemery, @bdraco, mind taking a look at this pull request as it has been labeled with an integration (mqtt) you are listed as a code owner for? Thanks!

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