feat: Loki remote write

[shuiyisong]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

This pr mainly modifies src/servers/src/http/event.rs file:

1. Add Loki remote write endpoint
2. Move HTTP result structs to a dedicated folder

use Alloy config like the following to remote write to GreptimeDB using Loki remote write endpoint

loki.write "local_loki" {
    endpoint {
        url = "http://localhost:4000/v1/loki/api/v1/push"
    }
}

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

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[codecov]

Codecov Report

Attention: Patch coverage is 85.02415% with 31 lines in your changes missing coverage. Please review.

Project coverage is 83.76%. Comparing base (0e0c4fa) to head (f7ad3af).
Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4941      +/-   ##
==========================================
- Coverage   84.07%   83.76%   -0.32%     
==========================================
  Files        1143     1148       +5     
  Lines      211918   214156    +2238     
==========================================
+ Hits       178172   179382    +1210     
- Misses      33746    34774    +1028     

[killme2008]

I think we can consume the labels to avoid cloning in the loop body.

Suggested change

	for (k, v) in labels.iter() {
	for (k, v) in labels.into_iter() {

[shuiyisong]

One Loki push request contains multiple log entries. The labels are request-level attributes, so these would be appended to each row of log entry. Hence the iter() instead of into_iter().