Implement Bool8

[chmp]

#211

Details from the PR defining the extension type:

Bool8 represents a boolean value using 1 byte (8 bits) to store each value instead of only 1 bit as in the original Arrow Boolean type. Although less compact than the original representation, Bool8 may have better zero-copy compatibility with various systems that also store booleans using 1 byte.

• Extension name: arrow.bool8.
• The storage type of this extension is Int8 where:
  • false is denoted by the value 0.
  • true can be specified using any non-zero value. Preferably 1.
• Extension type parameters: This type does not have any parameters.
• Description of the serialization: Metadata is an empty string.

Impls:

• Test that bool can serialized / deserialized into I8 arrays
• Test that any non-zero value is deserialized as true
• Implement extension type
• Add docs how to use the extension type with overwrites