PMM-14858 Skip built-in ClickHouse scrape for external instances

[4nte]

PMM-14858

Feature build

PMM kept scraping 127.0.0.1:9363 even when an external ClickHouse was configured or PMM_DISABLE_BUILTIN_CLICKHOUSE was set.

Introduced new ClickHouseParams model struct that validates and facilitates CH parameters.

ClickHouse scraping is skipped when any of the following are true:

1. ExternalVM is on. (old behavior)
2. PMM_DISABLE_BUILTIN_CLICKHOUSE is set. (new)
3. PMM_CLICKHOUSE_ADDR points to an external host. (new)

Addditionally, we log a warning when PMM_DISABLE_BUILTIN_CLICKHOUSE is not set and PMM_CLICKHOUSE_ADDR is an external address (not 127.0.0.1) to warn user about unwanted drift in their configuration.

[codecov]

Codecov Report

❌ Patch coverage is 77.41935% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.25%. Comparing base (6ecb7d5) to head (d3844dc).

Files with missing lines	Patch %	Lines
managed/cmd/pmm-managed/main.go	0.00%	13 Missing ⚠️
managed/services/victoriametrics/prometheus.go	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v3    #5369      +/-   ##
==========================================
+ Coverage   43.18%   43.25%   +0.06%     
==========================================
  Files         413      413              
  Lines       42253    42258       +5     
==========================================
+ Hits        18247    18277      +30     
+ Misses      22140    22115      -25     
  Partials     1866     1866              

Flag	Coverage Δ
managed	42.39% <77.41%> (+0.13%)	⬆️

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[CLAassistant]

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