httpclient: add composable HTTP client with layered RoundTripper chain

[mmatczuk]

Introduce a new internal/httpclient package that provides a production-ready
HTTP client built as a chain of http.RoundTripper decorators. The client is
configured via benthos service.ConfigField specs and assembled from parsed
YAML configuration.

RoundTripper chain (outermost to innermost):
  - Tracing: OpenTelemetry spans per request with method, URL, status attributes
  - Max response body: io.LimitReader cap on response bodies
  - Retry: exponential backoff with jitter, Retry-After header support,
    configurable status code classification (retry/drop/success), adaptive
    429-only mode as default, body replay via GetBody
  - TPS rate limit: token bucket via golang.org/x/time/rate
  - Metrics: benthos metrics (request duration, count, errors, active gauge)
    with method and status class labels
  - Logging: structured JSON access logs with configurable level and
    request/response body capture with prefix sniffing
  - Auth: bearer token and benthos signer (OAuth1, basic_auth, JWT)
  - Base transport: net/http.Transport with full HTTP/2 config, TLS,
    proxy, connection pooling, and dialer settings

Key design decisions:
  - Each layer is a no-op passthrough when not configured (zero allocation)
  - RetryConfig is a Go-only API (not YAML-exposed) for consumer control
    over retry/drop/success status codes; YAML exposes only 429 backoff
  - H2TransportConfig maps directly to net/http.HTTP2Config with validation
  - MaxIdleConnsPerHost defaults to GOMAXPROCS+1 instead of Go's default of 2
  - Response body drain on retry capped at 1MB to prevent stalling
  - Backoff jitter uses full-range [-delay/2, +delay/2] distribution

[claude]

Use %w instead of %v to preserve the error chain for errors.Is/errors.As upstream. Per project error handling patterns: "Use %w for wrapping (allows errors.Is/errors.As upstream). Use %v only when you intentionally want to break the error chain."

[claude]

Commits

1. The jira: switch to httpclient commit mixes documentation changes (docs/modules/components/pages/processors/jira.adoc) with code changes. Per commit policy, documentation changes must be in a separate commit from code changes in multi-commit PRs.

Review
The PR introduces a well-structured internal/httpclient package with layered RoundTripper chain (tracing, retry, rate limiting, metrics, logging, auth, base transport) and migrates the Jira processor to use it, removing the old jirahttp/http_metrics package and DoRequestWithRetries helper.

1. New code in internal/impl/jira/jirahttp/client.go uses %v instead of %w in fmt.Errorf, breaking the error chain. See inline comment.

[claude]

Commits
LGTM

Review
This PR introduces a well-designed internal/httpclient package with a composable RoundTripper chain (tracing, max body, retry, TPS rate limit, metrics, logging, auth, base transport), then migrates the Jira processor to use it — removing the old http_metrics and jira_helper packages.

LGTM