Support saved configuration for `tyger buffer` commands

[johnstairs]

Save values for --block-size and --dop in a file in $XDG_CONFIG_HOME. These values, if present, would become the default.

We could also support different values per hostname:

{
    "myeastusaccount.blob.core.windows.net": {
        "read": {
            "dop": 32
        },
        "write": {
            "dop": 64",
            "blockSize": "16MiB"
        }
    },
    "mywestusaccount.blob.core.windows.net": {
        "dop": 512
        "blockSize": "4MiB"
    },
    "*": {
        "dop": 32
        "blockSize": "16MiB"
    }
}