Agent configuration databind variables support

[varas]

This PR adds secrets management support for the infrastructure agent configuration file.

Therefore same variables section as the one defined for integrations is available.

IE:

variables:
  creds:
    vault:
      http:
        url: http://my.vault.host/v1/newengine/data/secret
        headers:
          X-Vault-Token: my-vault-token

proxy: https://${creds.user}:${creds.password}@myproxy.url

This also removes AgentMode type, as databind is not handling this well.

[coveralls]

Pull Request Test Coverage Report for Build 415587773

• 41 of 74 (55.41%) changed or added relevant lines in 8 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.04%) to 58.076%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/plugins/linux/hostinfo.go	0	1	0.0%
pkg/integrations/legacy/runner.go	0	1	0.0%
pkg/databind/pkg/databind/binder.go	0	3	0.0%
pkg/databind/pkg/databind/config.go	17	24	70.83%
pkg/databind/pkg/databind/replacer.go	4	11	36.36%
pkg/config/config.go	16	30	53.33%

Files with Coverage Reduction	New Missed Lines	%
pkg/config/config.go	1	70.71%
pkg/integrations/v4/logs/cfg_watcher.go	4	67.5%

Totals
Change from base Build 415536125:	-0.04%
Covered Lines:	11362
Relevant Lines:	19564

---

💛 - Coveralls

[carlosroman]

Nice changes. Will add a note somewhere about decoupling the databind providers.