fix: use uint8 instead of int8 for instruction serialization (#79)

We have some instructions to update product metadata that have size
of more than int8 max (127) and the on-chain program reads them as
u8. So we are switching to UInt8 instead.

Author: ali-bahjati

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "@pythnetwork/client",
-  "version": "2.21.0",
+  "version": "2.21.1",
   "description": "Client for consuming Pyth price data",
   "homepage": "https://pyth.network",
   "main": "lib/index.js",

package.json

@@ -93,9 +93,9 @@ export class PythOracleInstructionCoder implements InstructionCoder {
     if (methodName === 'updProduct' || methodName === 'addProduct') {
       let offset = 0
       for (const key of Object.keys(ix.productMetadata)) {
-        offset += buffer.subarray(offset).writeInt8(key.length)
+        offset += buffer.subarray(offset).writeUInt8(key.length)
         offset += buffer.subarray(offset).write(key)
-        offset += buffer.subarray(offset).writeInt8(ix.productMetadata[key].length)
+        offset += buffer.subarray(offset).writeUInt8(ix.productMetadata[key].length)
         offset += buffer.subarray(offset).write(ix.productMetadata[key])
       }
       if (offset > MAX_METADATA_SIZE) {