schema: Introduce DefaultValue to AttributeSchema

[radeksimko]

Background

Currently, when providing completion or hover, we use the attribute type.

There are many cases where an attribute can have a default value. e.g. Terraform provider schemas, or variables (exposed as attributes within module blocks). These defaults are already used to infer types when assembling attribute schemas, but the AttributeSchema definition doesn't hold these default values at all.

Proposal

• Introduce DefaultValue cty.Value to AttributeSchema
• Update CandidatesAtPos() to complete the default value on attribute completion
• Update HoverAtPos() to show default value in hover data