Fix schema for V0 HttpConfig headers

[yuri-tceretian]

Currently, the field Headers is a map[string]Header where value is the following type

alerting/http/v0mimir1/config.go

Lines 318 to 322 in 11bc3c6

	type Header struct {
	Values []string `yaml:"values,omitempty" json:"values,omitempty"`
	Secrets []commoncfg.Secret `yaml:"secrets,omitempty" json:"secrets,omitempty"`
	Files []string `yaml:"files,omitempty" json:"files,omitempty"`
	}

The schema definition is not correct as it represents the value as just map[string]string.

This introduces two problems:

1. Secrets in headers are not properly discovered\masked
2. Headers are not properly displayed.

[yuri-tceretian]

Proposed solution:

• update schema to support a new element type key_value_object_map. Together with SubformOptions that will let us specify value schema they will let us specify complex maps.
• update UI to correctly render the combination of fields.
• change schema for headers.
• update secrets discovery to support wildcard, so secrets could be represented like http_config.headers.*.secrets
• update secrets encryption\decryption to handle wildcard.