[Addon] enhance loki & prom addon

[Somefive]

Signed-off-by: Yin Da [email protected]

Description of your changes

prometheus-server

1. Support configuring scrapeInterval.
2. The resource configuration is only applied for prometheus-server container. The thanos sidecar container will not be enforced by that. (Solves The requested resource of the sidecar container of the Prometheus is too big #555)

loki

1. Support agentOnly parameter to skip the installation of loki.
2. Support lokiURL parameter to set the endpoint that agent needs to sink to.
3. The default agent mode is changed from "" (no agent install) to "vector".

How has this code been tested?

Checklist

I have:

• Title of the PR starts with type (e.g. [Addon] , [example] or [Doc]).
• Updated/Added any relevant documentation and examples.
• New addon should be put in experimental.
• Update addon should modify the version in metadata.yaml to generate a new version.

Verified Addon promotion rules

If this pr wants to promote an experimental addon to verified, you must check whether meet these conditions too:

• This addon must be tested by addon's e2e-test to guarantee this addon can be enabled successfully.
• This addon must have some basic but necessary information.
  • An accessible icon url and source url defined in addon's metadata.yaml.
  • A detail introduction include a basic example about how to use and what's the benefit of this addon in README.md.
  • Also provide an introduction in KubeVela documentation.
  • It's more likely to be accepted if useful examples provided in example dir.

[Somefive]

/cc @barnettZQG