function generateBlockID(blockIDPrefix, blockIndex) {
    // To generate a 64 bytes base64 string, source string should be 48
    const maxSourceStringLength = 48;
    // A blob can have a maximum of 100,000 uncommitted blocks at any given time
    const maxBlockIndexLength = 6;
    const maxAllowedBlockIDPrefixLength = maxSourceStringLength - maxBlockIndexLength;
    if (blockIDPrefix.length > maxAllowedBlockIDPrefixLength) {
        blockIDPrefix = blockIDPrefix.slice(0, maxAllowedBlockIDPrefixLength);
    }
    const res = blockIDPrefix +
        padStart(blockIndex.toString(), maxSourceStringLength - blockIDPrefix.length, "0");
    return base64encode(res);
}
Generate a 64 bytes base64 block ID string.

@param blockIndex -