identity: add configurable User-Agent header in requests

atproto/identity/base_directory.go

···
 	//
 	// The intended use-case for this flag is as an optimization for services which do not care about handles, but still want to use the `Directory` interface (instead of `ResolveDID`). For example, relay implementations, or services validating inter-service auth requests.
 	SkipHandleVerification bool
+	// User-Agent header for HTTP requests. Optional (ignored if empty string).
+	UserAgent string
 }
 
 var _ Directory = (*BaseDirectory)(nil)

atproto/identity/did.go

···
 	if err != nil {
 		return nil, fmt.Errorf("constructing HTTP request for did:web resolution: %w", err)
 	}
+	if d.UserAgent != "" {
+		req.Header.Set("User-Agent", d.UserAgent)
+	}
+
 	resp, err := d.HTTPClient.Do(req)
 
 	// look for NXDOMAIN
···
 	if err != nil {
 		return nil, fmt.Errorf("constructing HTTP request for did:plc resolution: %w", err)
 	}
+	if d.UserAgent != "" {
+		req.Header.Set("User-Agent", d.UserAgent)
+	}
+
 	resp, err := d.HTTPClient.Do(req)
 	if err != nil {
 		return nil, fmt.Errorf("%w: PLC directory lookup: %w", ErrDIDResolutionFailed, err)

atproto/identity/directory.go

···
 	"time"
 
 	"github.com/bluesky-social/indigo/atproto/syntax"
+
+	"github.com/carlmjohnson/versioninfo"
 )
 
 // Ergonomic interface for atproto identity lookup, by DID or handle.
···
 		TryAuthoritativeDNS: true,
 		// primary Bluesky PDS instance only supports HTTP resolution method
 		SkipDNSDomainSuffixes: []string{".bsky.social"},
+		UserAgent:             "indigo-identity/" + versioninfo.Short(),
 	}
 	cached := NewCacheDirectory(&base, 250_000, time.Hour*24, time.Minute*2, time.Minute*5)
 	return &cached

atproto/identity/handle.go

···
 	if err != nil {
 		return "", fmt.Errorf("constructing HTTP request for handle resolution: %w", err)
 	}
+	if d.UserAgent != "" {
+		req.Header.Set("User-Agent", d.UserAgent)
+	}
 
 	resp, err := d.HTTPClient.Do(req)
 	if err != nil {