funcs: Generate function signatures for Terraform v1.5

[radeksimko]

We can merge this when the release of v1.5 is imminent or when we're certain that these signatures won't change.

10c10
< func v1_4_0_Functions() map[string]schema.FunctionSignature {
---
> func v1_5_0_Functions() map[string]schema.FunctionSignature {
739a740,743
> 		},
> 		"plantimestamp": {
> 			ReturnType:  cty.String,
> 			Description: "`plantimestamp` returns a UTC timestamp string in [RFC 3339](https://tools.ietf.org/html/rfc3339) format, fixed to a constant time representing the time of the plan.",
991a996,1009
> 		},
> 		"strcontains": {
> 			Params: []function.Parameter{
> 				{
> 					Name: "str",
> 					Type: cty.String,
> 				},
> 				{
> 					Name: "substr",
> 					Type: cty.String,
> 				},
> 			},
> 			ReturnType:  cty.Bool,
> 			Description: "`strcontains` takes two values: a string to check and an expected substring. The function returns true if the string has the substring contained within it.",

[dbanck]

Nice! Thanks for tweaking the script to work with pre-releases!