Add missing availability annotations to fix iOS, watchOS, and tvOS builds (#233)

### Motivation

Currently this package doesn't build for the non-macOS Apple platforms.
For example, compiling for iOS fails with:

- `error: 'some' return types are only available in iOS 13.0.0 or newer`
- `error: Concurrency is only available in iOS 13.0.0 or newer`

Looks like a few types were missing the availability annotations when
this package switched to using them and dropping the minimum platform
requirement.

### Modifications

- Add missing annotations to restore build.

### Results

- Package now builds on iOS, tvOS, and watchOS.
- `struct NameConstraints`, `struct NameConstraintsValue` are marked
only available on iOS 13+ etc.
- `protocol OCSPRequester` is marked only available on iOS 13+ etc.

Author: simonjbeaumont

Sources/X509/Extension Types/NameConstraints.swift

@@ -23,6 +23,7 @@ import SwiftASN1
 /// Restrictions are defined in terms of both permitted and forbidden subtrees. The forbidden trees
 /// are consulted first, and if a name is matched in a forbidden tree then it does not matter whether
 /// the same name is also matched in a permitted tree.
+@available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, *)
 public struct NameConstraints {
     public struct DNSNames: Hashable, Sendable, Collection, ExpressibleByArrayLiteral, CustomStringConvertible {
         public typealias Element = String
@@ -689,10 +690,13 @@ extension Hasher {
     }
 }
 
+@available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, *)
 extension NameConstraints: Hashable {}
 
+@available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, *)
 extension NameConstraints: Sendable {}
 
+@available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, *)
 extension NameConstraints: CustomStringConvertible {
     public var description: String {
         var elements: [String] = []
@@ -712,6 +716,7 @@ extension NameConstraints: CustomStringConvertible {
     }
 }
 
+@available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, *)
 extension NameConstraints: CustomDebugStringConvertible {
     public var debugDescription: String {
         return "NameConstraints(\(String(describing: self)))"
@@ -743,6 +748,7 @@ extension NameConstraints: CertificateExtensionConvertible {
 
 // MARK: ASN1 Helpers
 @usableFromInline
+@available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, *)
 struct NameConstraintsValue: DERImplicitlyTaggable {
     @inlinable
     static var defaultIdentifier: ASN1Identifier {

Sources/X509/OCSP/OCSPPolicy.swift

@@ -25,6 +25,7 @@ import Foundation
 // Swift CI has implicit concurrency disabled
 import _Concurrency
 
+@available(macOS 10.15, iOS 13, watchOS 6, tvOS 13, *)
 public protocol OCSPRequester: Sendable {
     /// Called with an OCSP Request.
     ///