Config: Avoid emitting integers in objects using exponent notation

[justinvp]

Config values that are objects are represented in memory as JSON strings. When a config map is being saved to a file, object values are first unmarshaled from JSON to interface{} and then the entire config map is marshaled to YAML (or JSON) and saved to disk. When an object value is unmarshaled from JSON, any numbers in the JSON string were being implicitly unmarshaled as float64, which resulted in some numbers in the nested objects being emitted in YAML using exponent notation (e.g. a number 12321123131 in an object value was being saved in the YAML as 1.2321123131e+10). To address this, when unmarshaling the JSON for an object value, first try to unmarshal any numbers as int64, falling back to float64.

Fixes #6787

Before

Running pulumi config set --path parent.nested 12321123131 resulted in Pulumi.<stack>.yaml having:

config:
  proj:parent:
    nested: 1.2321123131e+10

After

Running pulumi config set --path parent.nested 12321123131 resulted in Pulumi.<stack>.yaml having:

config:
  proj:parent:
    nested: 12321123131

[stack72]

Thanks so much for this! This is a great bug fix