feat: capture x-client header for analytics (#468)

### Context
We recently added the ability to pass custom headers to identify which
Storacha client uses the Upload API (see:
storacha/upload-service#215). Now we need to
capture the custom header for analytics.

### Changes
- Updated the `ucan-invocation-handler` to log additional information if
the `X-Client` custom header is present.
- The logs are structured in JSON format that's easy to query in
CloudWatch Logs.
- The `clientId` field will contain the actual value of the `X-Client`
header which indicates the name and version of the Storacha Client +
service/project it is integrated to, e.g: `Storacha/X (js; MCP/Y)` -
where `X` = major version of the Storacha Client, and `Y` = major
version of the service/project integration.
- The `requestId` allows us to correlate this specific log entry with
other logs from the same request, which helps debug issues.
- Then we build analytics based on CloudWatch Logs Insights using some
query like this:
   ```
   fields @timestamp, clientId
   | filter message = "Client request"
   | stats count(*) as requestCount by clientId
   | sort requestCount desc
   ```

Related to: storacha/mcp-storage-server#18

Author: fforbeck

upload-api/functions/ucan-invocation-router.js

@@ -118,6 +118,20 @@ export const knownWebDIDs = {
  * @param {import('aws-lambda').APIGatewayProxyEventV2} request
  */
 export async function ucanInvocationRouter(request) {
+  try {
+      // Capture X-Client custom header for analytics
+      const clientId = Object.entries(request.headers)
+      .find(([key]) => key.toLowerCase() === 'x-client')?.[1] ?? 'Storacha/?'
+      console.log(JSON.stringify({
+        message: 'Client request',
+        clientId,
+        requestId: request.requestContext?.requestId || 'unknown',
+          timestamp: new Date().toISOString()
+      }))
+  } catch (error) {
+    console.error(error)
+  }
+
   const {
     storeTableName,
     storeBucketName,
@@ -205,7 +219,7 @@ export async function ucanInvocationRouter(request) {
       credentials: {
         accessKeyId: carparkBucketAccessKeyId,
         secretAccessKey: carparkBucketSecretAccessKey,
-      }, 
+      },
     }),
     createBlobsStorage(AWS_REGION, storeBucketName),
   )
@@ -267,7 +281,7 @@ export async function ucanInvocationRouter(request) {
   const storageProviderTable = createStorageProviderTable(AWS_REGION, storageProviderTableName, options)
   const routingService = createRoutingService(storageProviderTable, serviceSigner)
 
-  
+
   let audience // accept invocations addressed to any alias
   const proofs = [] // accept attestations issued by any alias
   if (UPLOAD_API_ALIAS) {
@@ -380,7 +394,7 @@ export const fromLambdaRequest = (request) => ({
   body: Buffer.from(request.body || '', 'base64'),
 })
 
-function getLambdaEnv () {
+function getLambdaEnv() {
   return {
     storeTableName: mustGetEnv('STORE_TABLE_NAME'),
     storeBucketName: mustGetEnv('STORE_BUCKET_NAME'),