feat: add MQTT keepalive_interval config to allow custom value per bridge

[rina23q]

Proposed changes

As per the request in #3153, add <cloud>.bridge.keepalive_interval to the tedge config settings.
Example usage:

tedge config set c8y.bridge.keepalive_interval 60s

With mosquitto, the value will be shown explicitly in c8y-bridge.conf as :

### Bridge
....
keepalive_interval 60

Using 60s as default as I found 60 seconds is the default for both mosquitto and rumqttc.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

#3153

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[codecov]

Codecov Report

Attention: Patch coverage is 69.38776% with 15 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../tedge_config/src/tedge_config_cli/tedge_config.rs	23.07%	0 Missing and 10 partials ⚠️
crates/core/tedge/src/bridge/config.rs	92.85%	0 Missing and 1 partial ⚠️
crates/core/tedge/src/cli/connect/command.rs	0.00%	0 Missing and 1 partial ⚠️
crates/core/tedge_mapper/src/aws/mapper.rs	0.00%	1 Missing ⚠️
crates/core/tedge_mapper/src/az/mapper.rs	0.00%	1 Missing ⚠️
crates/core/tedge_mapper/src/c8y/mapper.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

📢 Thoughts on this report? Let us know!

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
556	0	2	556	100	1h29m27.124075s

[didier-wenzek]

Approved.

Unrelated to this PR:

• The changes for c8y, az and aws bridges are so similar that it sounds as a failure to be DRY. Actually, the main point is to configure a mosquitto bridge and one should have a mosquitto::bridge abstraction, not an abstraction per cloud.

[albinsuresh]

LGTM

[albinsuresh]

Optional:

Suggested change

	keepalive_interval: SecondsOrHumanTime,
	keep_alive_interval: SecondsOrHumanTime,

[albinsuresh]

I take that back, after noticing that the term keepalive is kinda like the norm across protocols like HTTP and GRPC.